Nuclear reactors, such as pressurized water reactors and boiling water reactors, generate nuclear power through the use of nuclear fuel assemblies housed in a reactor core. The fuel assemblies are comprised of elongated hollow metallic tubular fuel rods that contain pellets of enriched uranium dioxide material. The hollow metallic fuel tubular rods, commonly known in the nuclear industry as cladding, prevent the escape of materials, such as uranium dioxide and fission gasses, from the interior of the fuel rod. The cladding is generally configured from an alloy of differing metals including zirconium. These alloys are used principally because of the low neutron capture characteristic of zirconium. As a consequence of the low neutron capture characteristic, these alloys have been used extensively in the nuclear industry as the nuclear reaction in the reactor is least hindered by such use.
Fuel rods that have been irradiated and have had the enriched uranium dioxide content depleted due to operation of the reactor core, are often stored in pools of cooling water to remove decay heat. As time passes, materials can collect on an exterior surface of the zirconium based alloys. Moreover, the zirconium alloy may warp and swell on the exterior portion of the fuel rod, creating a further layer of material on the underlying sound zirconium alloy substrate. The material which collects on the exterior of the zirconium based alloy is commonly known as Chalk River Unidentified Deposit or “CRUD.” The CRUD located on an exterior of the fuel rods of a fuel assembly is generally made of solid particles, agglomerated together, that can be strongly attached to the underlying substrate. Due to the closeness of the CRUD to the activated uranium dioxide material in the fuel assemblies, the CRUD is usually highly radioactive. The CRUD on the fuel rods can become dislodged from the underlying zirconium alloy substrate by flowing water that the fuel assemblies are immersed in. Consequently, CRUD can enter the piping of the water systems in the nuclear power plant and travel along these systems causing unintended irradiation of personnel in plant areas that are normally not radiologically active.
Although CRUD is a non-homogenous material, CRUD has been found to be generally made of several elemental components. The major components of CRUD can include, for example, iron, cobalt, zinc, silica, chrome and manganese. As nuclear plant fuel performance is influenced by CRUD deposition during normal plant operation as well as sequence and economics of refueling and maintenance outages, it is necessary to analyze fuel assemblies for the presence of CRUD to determine the nature and amount of the deposits. For example, if it is determined that the nuclear fuel rods have a highly radioactive CRUD layer that may become easily dislodged, then a worker radiological concern exists where the fuel rods may be required to be cleaned. This cleaning process is usually conducted by a number of means, ultrasonically or chemically cleaning the exterior of the fuel rods to remove the loose CRUD buildup.
To perform analysis of crud deposited on the fuel assemblies, samples must be taken by mechanically scraping the exterior of the fuel rods. The systems used to perform this mechanical scraping include a rigid member that the fuel rod is pressed against, thereby shearing the loose CRUD from the rest of the nuclear fuel rod when the fuel rod is moved over the rigid member.
Devices for mechanical scraping fuel rods may be divided into two sub-classes. Manually operated devices may be used to remove the CRUD from the exterior surface of the nuclear fuel rods. Such devices consist of a scraping head at the end of a shaft, wherein the scraping head is used to dislodge loose CRUD from the exterior surface of the fuel rod. Automated devices may also be used to remove deposits from irradiated fuel rods. The fuel rods are scraped by a remote control scraping device, wherein the scraped sample is conveyed to an internal reservoir.
Previous devices of both sub-categories have several drawbacks that limit the effectiveness of the removal devices. Existing manual devices are only used to remove materials from an outside of a fuel rod that are easily dislodgable. CRUD deposits that are attached to the nuclear fuel rod more tenaciously are not able to be removed using existing manual tools. As a result, the manual tools used do not provide an accurate representation of CRUD materials that may be present in the entire depth of the fuel rod as the sampling occurs only on an exterior subsection of the total CRUD deposit. Mechanized devices, however, scrape the nuclear fuel rods such that the entire CRUD deposit is removed from a portion of the fuel rod surface, as well as warped zirconium alloy on the external portion of the nuclear fuel rod. Removal of any warped zirconium alloy on an external portion of the nuclear fuel rod damages the fuel rod. This necessitates an extensive engineering analysis to determine if the pressure retaining capabilities of the fuel rod have been severely compromised. When the material constituents of the exterior of the fuel rod are sampled after using a mechanized device, zirconium alloy inappropriately removed from the rod will skew the material analysis results. Another drawback of existing mechanized devices for removing CRUD deposits on the exterior of fuel rods is that these devices are economically expensive to produce and often require significant maintenance for operation. Additionally, performing maintenance activities on radioactive components of the mechanized devices increases worker radiation exposure.
There is therefore a need to provide a CRUD removal tool, which is cost effective for nuclear reactor operators.
There is also a need for a CRUD removal tool, which will limit potential damage to nuclear fuel rods during removal of CRUD.
There is also a need for an analysis method for CRUD deposits, which will adequately categorize CRUD obtained from scraped fuel rods.